Скачать или смотреть Erasure Coding vs RAID [EXPLAINED]

Erasure coding and RAID (Redundant Array of Independent Disks) are both storage technologies used to protect data from disk failures by providing redundancy. However, they achieve redundancy in different ways, and each has its advantages and trade-offs.



RAID (Redundant Array of Independent Disks):


RAID uses various configurations, known as RAID levels, to provide redundancy and performance improvements. Common RAID levels include RAID 1, RAID 5, and RAID 6:


RAID 1 (Mirroring):


In RAID 1, data is mirrored across two or more disks. Each disk in the array contains an identical copy of the data. If one disk fails, the data is still available from the mirror.
RAID 5 (Striping with Parity):


RAID 5 stripes data across multiple disks and includes parity information. If one disk fails, the parity information can be used to reconstruct the lost data. RAID 5 requires a minimum of three disks.
RAID 6 (Striping with Dual Parity):


RAID 6 is similar to RAID 5 but includes dual parity, allowing the array to tolerate the failure of two disks simultaneously. RAID 6 requires a minimum of four disks.
Erasure Coding:


Erasure coding is an advanced form of redundancy that breaks data into fragments, adds redundancy information (parity), and distributes the fragments and parity across multiple disks. Common erasure coding methods include Reed-Solomon and Systematic Reed-Solomon:


Reed-Solomon Coding:


Reed-Solomon coding divides data into blocks and adds parity blocks, providing a level of fault tolerance. The number of parity blocks determines the level of redundancy.
Systematic Reed-Solomon Coding:


This is an extension of Reed-Solomon coding where the original data is included as part of the encoded data, reducing the need for decoding during data retrieval.
Comparison:


Fault Tolerance:


RAID: The fault tolerance in RAID depends on the RAID level. RAID 1 can tolerate the failure of one disk, RAID 5 can tolerate one disk failure, and RAID 6 can tolerate two simultaneous disk failures.
Erasure Coding: The fault tolerance in erasure coding depends on the specific coding scheme. For example, a Reed-Solomon code with
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n data blocks and
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m parity blocks can tolerate up to
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m disk failures.
Storage Efficiency:


RAID: Mirroring (RAID 1) has a 1:1 storage efficiency since each data block is mirrored. RAID 5 and RAID 6 have better storage efficiency as they use parity, but they still require more raw capacity than the usable capacity.
Erasure Coding: Erasure coding generally offers better storage efficiency than RAID because it can provide fault tolerance with less redundancy.
Rebuild Time:


RAID: Rebuilding a RAID array after a disk failure can take time, especially for large capacity disks.
Erasure Coding: Erasure coding may also have to rebuild data, but the rebuild time can be more efficient as it involves only the data and parity blocks affected by the failure.
Write Performance:


RAID: RAID 1 typically has good write performance, but RAID 5 and RAID 6 may experience a performance penalty due to parity calculations during writes.
Erasure Coding: Erasure coding can also introduce a write penalty due to the need to calculate and update parity information.
Use Cases:


RAID: Commonly used in traditional storage systems and arrays.
Erasure Coding: Commonly used in distributed storage systems and cloud storage where storage efficiency and fault tolerance are critical.
In summary, both erasure coding and RAID serve the purpose of providing fault tolerance, but the choice between them depends on specific use cases, performance requirements, and storage efficiency considerations. Erasure coding is often favored in modern distributed storage systems, especially in cloud environments, where efficiency and scalability are key considerations. RAID, on the other hand, is still widely used in traditional storage systems.